What causes a bad Whopper?

I’m talking about the candy. I’ll be munching along, enjoying the crispety-crunchety goodness, when suddenly, it happens: I bite into a bad one. You know instantly when it happens, but it’s too late to do much about it. You just suffer with the chewy wrongness of the thing. What is going on inside that chocolate covered ick?

Bonus question: Is there a way to tell one is bad just by looking at it? Some way to save us from the suffering? ~